The conversion of a single ultraviolet photon into two visible photons with the result that the quantum efficiency of luminescence exceeds unity is termed quantum splitting. Quantum splitting materials are very desirable for use as phosphors for lighting applications, such as fluorescent lamps. A suitable quantum splitting material can in principle produce a significantly brighter light source. Quantum splitting has been demonstrated previously in fluoride-base materials. A material comprising 0.1% Pr.sup.3+ in a matrix of YF.sub.3 has been shown to generate more than one visible photon for every absorbed UV photon when excited with 185 nm radiation. The measured quantum efficiency of this material was 145%.+-.15%, and thus greatly exceeded unity. The critical conditions that yield quantum splitting by the Pr.sup.3+ ion have only been achieved in fluoride-base materials. However, fluoride-base compounds do not have sufficient stability to permit their utilization as a phosphor in fluorescent lamps, because they are known to react with the Hg vapor that is used in such lamps to provide the UV radiation and form a material that does not exhibit quantum splitting.
Therefore, it is desirable to develop materials that exhibit quantum splitting behavior, but that are also stable and suitable for use in current technology lamps (e.g. Hg-containing fluorescent lamps), such as metal oxides.